Because of their high volumetric efficiency and thus their small size, multilayer capacitors (MLC's) are the most widely used form of ceramic capacitors for thick film hybrid microelectronic systems. These capacitors are fabricated by stacking and cofiring thin sheets of ceramic substrate on which an appropriate electrode pattern is printed. Each patterned layer is offset from the adjoining layers in such manner that the electrode layers are exposed alternately at each end of the assemblage. The exposed edges of the electrode pattern are coated with a conductive material which electrically connects all the layers of the structure, thus forming a group of parallel connected capacitors within the laminated structure. Capacitors of this type are frequently referred to as monolithic capacitors.
The thin sheets of ceramic substrate used for the fabrication of MLC's are commonly referred to as "green tapes" and are comprised of a thin layer of finely divided dielectric particles which are bound together by an organic polymeric material. Unfired green tapes are prepared by slip casting a slurry of the dielectric particles dispersed in a solution of polymer, plasticizer and solvent onto a carrier such as polypropylene, Mylar.RTM. polyester film or stainless steel and then adjusting the thickness of the cast film by passing the cast slurry under a doctor blade.
A wide variety of capacitor materials can be fabricated in this way, including TiO.sub.2 -glass (BaSr)TiO.sub.3 (Ba,Pb)TiO.sub.3 -glass, PbZrO.sub.3 TiO.sub.3 --BaTiO.sub.3 --Pb monosilicate glass, BaTiO.sub.3 Bi.sub.2 O.sub.3, ZnO, BaTiO.sub.3 -Cd borosilicate glass, BaTiO.sub.3 --BaAl.sub.2 --SiO.sub.8, BaTiO.sub.3 --Pb.sub.2 Bi.sub.4 Ti.sub.5 O.sub.18 and countless others.
In the past, various polymeric materials have been employed as the binder for green tapes, e.g., poly(vinyl butyral), poly(vinyl acetate), poly(vinyl alcohol), cellulosic polymers such as methyl cellulose, ethyl cellulose, hydroxy ethyl cellulose, methylhydroxy ethyl cellulose, atactic polypropylene, polyethylene, silicon polymers such as poly(methyl siloxane), poly(methyl phenyl siloxane), polystyrene, butadiene/styrene copolymer, polystyrene, poly(vinyl pyrrolidone), polyamides, high molecular weight polyethers, copolymers of ethylene oxide and propylene oxide, polyacrylamides, and various acrylic polymers such as sodium polyacrylate, poly(lower alkyl acrylates), poly(lower alkyl methacrylates) and various copolymers and multipolymers of lower alkyl acrylates and methacrylates. Copolymers of ethyl methacrylate and methyl acrylate and terpolymers of ethyl acrylate, methyl methacrylate and methacrylic acid have been previously been used as binders for slip casting materials.
In large part because of the engineering characteristics of slip casting equipment, it is preferred that the ceramic dispersion (slip) for casting dielectric films fall within the viscosity range of 100-4000 cp and preferably 500-3000 cp (Brookfield LVT Viscometer No. 2 spindle, 6 rpm, 25.degree. C.). Within these viscosity limits, it is preferred to employ a casting slip which contains the highest possible amount of dielectric material and the least amount of polymeric binder and solvent. By this means, the amount of material which must be removed by pyrolysis can be kept to a minimum and problems with delamination of the layers are reduced. Heretofore, this has been a persistent problem for the reason that polymers which have sufficiently high molecular weight to provide adequate tape toughness and yet permit the use of relatively low amounts of binder are too high in viscosity. This is important in that the high viscosity of the polymer limits the amount of dielectric solids which can be used. In turn, this increases the ratio of organics to dielectric solids, which means that more organic materials must be pyrolyzed. On the other hand, polymers with low enough molecular weight to give suitably low viscosity and therefore higher amounts of dielectric solids yield layers which are too brittle.